1. Field of the Invention
The present invention relates to a black and white photothermographic material and an image forming method. More particularly, the invention relates to a high image quality black and white photothermographic material for medical use and an image forming method using the same.
2. Description of the Related Art
In recent years, in the medical field and the graphic arts field, there has been a strong desire for providing a dry photographic process from the viewpoints of protecting the environment and economy of space. Further, the development of digitization in these fields has resulted in the rapid development of systems in which image information is captured and stored in a computer, and then when necessary processed and output by transmitting it to a desired location. Here the image information is output onto a photosensitive material using a laser image setter or a laser imager, and developed to form an image at the location. It is necessary for the photosensitive material to be able to record an image with high-intensity laser exposure and that a clear black-tone image with a high resolution and sharpness can be formed. While various kinds of hard copy systems using pigments or dyes, such as ink-jet printers or electrophotographic systems, have been distributed as general image forming systems using such digital imaging recording materials, images on the digital imaging recording materials obtained by such general image forming systems are insufficient in terms of the image quality (sharpness, granularity, gradation, and tone) needed for medical images used in making diagnoses, and high recording speeds (sensitivity). These kinds of digital imaging recording materials have not reached a level at which they can replace medical silver halide film processed with conventional wet development.
Photothermographic materials utilizing organic silver salts are already known. Photothermographic materials generally comprise an image forming layer in which a reducible silver salt (for example, an organic silver salt), a photosensitive silver halide, and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder.
Photothermographic materials form a black silver image by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region. There is much literature in which photothermographic materials are described, and the Fuji Medical Dry Imager FM-DPL is an example of a medical image forming system that has been made commercially available.
Photothermographic materials using a silver salt of a nitrogen-containing heterocyclic compound as an organic silver salt and a hydrophilic binder such as gelatin are disclosed in U.S. Pat. No. 6,576,410.